This video presents a series of low back flexion exercises. These may be useful for people that have reduced mobility in bending forward, and/or stiffness related to standing or walking for most of the day, among other possible reasons.
If you have tried these stretches and have not made progress, or even experienced a worsening in symptoms: get assessed! Talk to a physical therapist. Find out where the hang up is and what else should be done. Many variables can be manipulated to get you the results you want, but knowledge of your specific circumstances may be necessary to develop a useful recipe.